Composition and method for producing an ultra-lightweight ceramic proppant

ABSTRACT

A pellet made from a mixture of natural occurring clays comprising porcelain clay, earthenware clay, and kaolin clay. In some embodiment, the pellet has an alumina content between about 5.5% and about 35%, an apparent specific gravity from about 2.10 to about 2.55 g/cc, and a bulk density of from about 1.30 to about 1.50 g/cc. The pellet is useful in hydraulic fracturing of oil and gas wells, and has greater conductivity than sand at pressures up to 8,000 psi as measured by Stim-Lab after 50 hours and 275° F. on Ohio Sandstone, in the presence of deoxygenated aqueous 2% solution of KCI.

This application is a continuation application of U.S. patentapplication Ser. No. 13/386,143. The entire disclosures of the precedingpatent application are incorporated by reference herein in theirentirety.

REFERENCES TO RELATED APPLICATIONS

This application is the continuation application of U.S. patentapplication Ser. No. 13/386,143, filed on Jan. 20, 2012, entitled“COMPOSITION AND METHOD FOR PRODUCING AN ULTRA-LIGHTWEIGHT CERAMICPROPPANT,” which is the National Stage of Int'l App'l No.PCT/US10/42980, filed Jul. 23, 2010, claiming priority to U.S.provisional application Ser. No. 61/228,562, filed Jul. 25, 2009, andChinese patent application serial No. 201010157346.4, filed Apr. 27,2010

BACKGROUND

Hydraulic fracturing is a technical operation designed to enhance oiland gas production. The process normally involves two steps. First, ahydraulic fracture is formed by injecting a fracturing fluid into thewell bore at a sufficient rate and pressure to cause the formationcracked thereby allowing the fluid enter and extend the crack further.Next, a solid propping agent or proppant is carried and placed into theformation to “prop” the fracture open and provide a conductive channelfor oil and gas. Oil and gas reservoirs with a low capacity for the flowof fluids usually require hydraulic fracturing to make them commerciallyviable.

Various types of materials can be used as propping agents, such as sand,resin-coated sand, and man-made ceramic proppant, depending on the typeof permeability or particle strength needed. A good propping agent musthave sufficient strength to resist crushing by the closure stresses ofthe formation. The deeper the well depth, the stronger the proppantneeds to be to resist crushing. The most commonly used proppant is sand,due to its vast availability and low cost. However, for deeperapplication, sand does not have sufficient strength to resist crushingdue to the closure stresses of the formation, and the permeability ofsand is often inadequate.

Sintered bauxite, a high-density proppant with apparent specificgravities about 3.50 g/cc, having an alumina content of about 83% isrecommended for use in well depths of greater than 15,000 feet.

Intermediate density proppant, e.g., apparent specific gravity fromabout 3.10 g/cc to 3.45 g/cc, has been found to have sufficient strengthto provide adequate permeability at intermediate depths and pressures,and is strong enough to withstand crushing at well depths from about8,000 to about 12,000 feet.

However, both high-density and intermediate-density proppants requirelarge pumping equipment, high viscosity fracturing fluids and highpumping rates to keep them in suspension during the fracturingoperation, and cause greater than normal wear on fluid carrying andpumping equipment.

Because of the disadvantages of high-density and intermediate-densityproppants, the efforts to get a lighter density and crush-resistantproppant by using lower alumina material have been taken since 1980s.Additionally, more and more slickwater fracturing and horizontal wellapplications require lightweight ceramic proppants.

A low density proppant is described in U.S. Pat. No. 4,522,731 and U.S.Pat. No. 5,120,455 to Lunghofer, using kaolin clay having a 50% aluminacontent. This low density proppant has an apparent specific gravity ofless than 3.0 g/cc.

Another lower density proppant, having an apparent specific gravity offrom 2.20 g/cc to 2.60 g/cc, is described in U.S. Pat. No. 5,188,175 toSweet, using raw material with an alumina content of from 25% to 40%.

An even lower density proppant is described in U.S. Pat. No. 7,036,591to Cannan, having the alumina content between about 40 and 60%, for usein shallow oil and gas wells. To achieve the specific gravity of fromabout 1.60 g/cc to about 2.10 g/cc, the firing time of the pellet isless than about 30 minutes and the goal is to sinter and obtaincrystallization without fully densifying the proppant.

Since most of fracturing operations are in the low depth range wheresand is used, it is desirable to provide a lightweight proppant whichcan be produced from inexpensive, low alumina content, e.g. a naturallyoccurring clay with alumina content less than 25% and which exhibitsconductivity greater than sand when used in low or medium depthapplications.

Reducing the alumina content of the particle generally reduces itsdensity. However, there is a real problem with the strength of theproppant when the alumina content gets too low. For this reason, thegoal of the industry has been set to develop a low density proppantwithout sacrificing strength.

The ultra-lightweight proppant of the present invention uses naturallyoccurring clays, such as porcelain clay, kaolin (or flint clay) as mainraw materials, and are less expensive than bauxite. Therefore, it haslower manufacturing cost per pound. Moreover, the naturally occurringclays have more availability than bauxite.

In addition, from end-user's point view in hydraulic fracturing, fewerpounds of ultra-lightweight proppant are required to fill a crack in theformation for a given propping application because of its lower bulkdensity as compared to high density or intermediate density proppants.This is a significant advantage since proppants are generally sold byweight.

Other advantages of the ultra-lightweight proppant include itscapability of using a lower viscosity fracturing fluid, being employedat more economical pumping rates, and causing less wear onfluid-carrying and pumping equipment than those required by high orintermediate density proppant.

SUMMARY OF THE INVENTION

This invention discloses an ultra-lightweight proppant made fromnaturally occurring clays having a alumina content from about 5.5% to35%, preferably from 5.5% to 25%, most preferably from 14% to 25%. Theproppant of this invention has an apparent specific gravity of fromabout 2.10 g/cc to about 2.55 g/cc, preferably 2.30 to 2.50 g/cc; and abulk density of from about 1.30 g/cc to about 1.50 g/cc, preferably from1.30 to 1.40 g/cc. The proppant of this invention exhibits greaterconductivity than sand at pressures up to 8,000 psi after 50 hours and275° F. on Ohio Sandstone, in the presence of deoxygenated aqueous 2%solution of KCI evaluated by of STIM-Lab.

In one embodiment, the proppant of this invention may be made from amixture comprising 5% to 85% of porcelain clay, 5% to 85% kaolin clayand 5% to 30% earthenware clay. The mixture has an alumina content fromabout 5.5% to 35%, preferably from 5.5% to 25%, most preferably from 14%to 25%.

In some preferred embodiment, the proppant of this invention has asilicon dioxide (SiO2) content between 89.5% and 69.5%, preferablybetween 81.5% to 69.5%.

This invention further discloses a sintered spherical pellet having asubstantially round and spherical shape that manures at least about 0.8on the Krumbein/Sloss Roundness and Sphericity Chart. The aluminacontent of said pellet is from about 5.5% to 35% percent, preferablyfrom 5.5% to 25%, and most preferably from 14% to 25%. The pellet ofthis invention has an apparent specific gravity of from about 2.10 g/ccto about 2.55 g/cc, preferably 2.30 to 2.50 g/cc; and a bulk density offrom about 1.30 g/cc to about 1.50 g/cc, preferably from 1.30 to 1.40g/cc.

This ultra-lightweight proppant can be employed in hydraulic fracturingas well as gravel packing media for sand control in oil and gas wells.It exhibits greater conductivity than sand at pressures up to 8,000 psiafter 50 hours and 275° F. on Ohio Sandstone, in the presence ofdeoxygenated aqueous 2% solution of KCI evaluated by of STIM-Lab.

In order to obtain the maximum strength of proppant particle, thepellets are sintered at firing time from about 75 minutes to about 960minutes, preferably from about 120 minutes to about 720 minutes, viatunnel kiln or rotary kiln. Generally, the firing temperature is fromabout 1150° C. to about 1380° C., preferably between 1200° C. to 1320°C.

This invention further discloses a method of fracturing a subterraneanformation which comprises injecting a hydraulic fluid into the formationat a rate and pressure sufficient to open a fracture therein, andinjecting into the fracture a fluid containing sintered sphericalpellets disclosed in this invention.

DETAILED DESCRIPTION

The ultra-lightweight proppant of this invention is in the form ofsolid, spherical pellets made from mixtures of naturally occurring clayshaving an alumina content from about 5.5% to about 35%, preferably from5.5% to 25%, most preferably from 14% to 25%. The proppant of thisinvention has an apparent specific gravity of from about 2.10 g/cc toabout 2.55 g/cc, preferably 2.30 to 2.50 g/cc; and a bulk density offrom about 1.30 g/cc to about 1.50 g/cc, preferably from 1.30 to 1.40g/cc. The proppant of this invention exhibits greater conductivity thansand at pressures up to 8,000 psi after 50 hours and 275° F. on OhioSandstone, in the presence of deoxygenated aqueous 2% solution of KCIevaluated by of STIM-Lab. All percentages used in this invention areweight percentages.

In one embodiment, the proppant of this invention may be made from amixture comprising 5% to 85% porcelain clay, 5% to 85% Kaolin clay and5% to 30% earthenware clay. The mixture has an alumina content fromabout 5.5% to 35%, preferably from 5.5% to 25%, most preferably from 14%to 25%.

Raw Materials

In accordance with this invention, one such raw material is porcelainclay. Porcelain clay is a fine soil usually in white color formed fromthe weathering of aluminous minerals, such as feldspar or china stone.The toughness, strength, and translucence of porcelain clay arise mainlyfrom the formation of glass and the mineral mullite within the firedbody at these high temperatures. Porcelain clay generally has an aluminacontent less than about 20% and a silica content greater than about 65%.The percentages are expressed herein on a dry weight basis.

The second raw material is used for this invention is earthenware clay.Earthenware clay is a very fine-grained soil that is plastic and stickywhen moist but hard when fired. Earthenware clay exhibits betterplasticity than porcelain clay and kaolin when mixed with water incertain proportions. When dry, the earthenware clay becomes firm andwhen fired in a kiln, permanent physical and chemical reactions occur.These reactions, among other changes, cause the earthenware clay to beconverted into a strong ceramic material. Because of these properties,especially the plasticity, earthenware clay is an excellent raw materialto be used for making green pellets of proppants. Earthenware claygenerally has an alumina content less than about 25% and a silicacontent greater than about 60%.

The third main raw material is kaolin. Kaolin, or china clay, generallyhas an alumina content about 40%, and a silica content about 45%. Inchemical composition, kaolin clay consists largely of kaolinite whichthe formula of the mineral is (Al₂O₃.2SiO₂.2H₂O). Kaolin is a highlyrefractory clay and has a melting point above 1700° C. Used by itself,kaolin is difficult to shape into pellets because of its poorplasticity, and also, because of its refractoriness, it is difficult tobe sintered by firing to a hard, dense particle. To make proppant,kaolin is seldom used by itself; therefore, porcelain and earthenwareclays have to be added to it to increase its workability and to lowerthe kiln temperature necessary to produce a hard and dense proppant.

Porcelain clay, earthenware clay and kaolin used in this invention arefrom Wanyao, Ningde City, Fujian Province in China (They have beenrecognized as “Ningde Porcelain Clay”, “Ningde Earthenware Clay” &“Ningde Kaolin” respectively). Another source of porcelain clay,earthenware clay and kaolin suitable to make this proppant is minded inWanwu, Fuan City, Fujian Province in China (They have been recognized as“Fuan Porcelain Clay”, “Fuan Earthenware Clay” & “Fuan Kaolin”respectively). Both Ningde and Fuan are places in China with longhistory of making ceramic products for household and industrialapplications. For example, Wangyao, literally means “the Kiln of Bowl”,is famous of making various household utensils, such as bowl, urn, andpottery, etc. The principal uses of Fuan clays are in the manufacture ofpottery, firebrick and of various products used in the metalworkingindustries, such as crucibles, saggers, retorts, etc. The amount of freesand (i.e. quartz) of both Ningde and Fuan clays is less than 2 weightpercent (hereinafter “w/o”).

To make the proppant of this invention, here kaolin can be replaced withflint-clay. Flint-clay is a calcined fireclay with high refractoriness,and should contain high percentages of silica and alumina, with aslittle as possible of such impurities as lime, magnesia, soda, andpotash, which lower the fusion point of the clay. Flint-clay generallyhas an alumina content about 45%, and a silica content greater thanabout 40%.

Because kaolin (or flint-clay) has a high enough alumina content, itshall be mixed with porcelain clay and earthenware clay composed largelyof silicon dioxide to obtain a mixture having a content of between 5.5%and 35% alumina, preferably from 5.5% to 25%, and most preferably from14% to 25%.

In one embodiment, the weight percentage of the raw material mixtures isas follows:

-   -   Porcelain Clay: 5%˜85%    -   Kaolin (and/or Flint-clay): 5%˜85%    -   Earthenware Clay: 5%˜30%

The main impurities of porcelain clay, earthenware clay and kaolin (orflint-clay) are compounds of titanium, iron, and small amounts ofpotassium and magnesium.

In some preferred embodiment, the raw material mixture may furthercomprises strengthening additives selected from titanium concentrate,zircon sand, aluminum oxide or combinations thereof. The weightpercentage of the strengthening additives is about 5% to about 10%.

The ultra-lightweight proppant disclosed in this invention may also bemade from other naturally occurring clays having a alumina content fromabout 5.5% to 35%, preferably from 5.5% to 25%, most preferably from 14%to 25%.

Term Introduction

For the purposes of this invention, the term “sand” is used to refer topremium quality “Ottawa” sand having a 20/40 mesh size.

The term “conductivity” used in this invention is to refer to width ofthe fracture multiplied by the permeability of the proppant pack. Theconductivity of a proppant under specific conditions of stress,temperature, corrosive environment and time is the single most importantmeasure of proppant's quality.

The conductivity of a packed proppant is defined as the permeability ofthe proppant pack multiplied by the width of the propped fracture and isusually stated in units of millidarci-feet (“md-ft”).

The term “permeability” as used herein is a measure of the ability ofmedia to transmit fluid through pore spaces. Generally, the data ofconductivity or permeability is one of the best indicators of its crushstrength, and it can provide valuable information about the performanceof proppant in the subterranean formation.

The term “sphericity” used in this invention is to measure of how closea proppant particle approaches the shape of a sphere. The term“roundness” as used herein is a measure of the relative sharpness ofproppant corners, or of proppant curvature. A chart developed byKrumbein and Sloss has been widely used in the visual estimation ofsphericity and roundness since 1963. Both sphericity and roundnessshould be determined, recorded, and average numbers should be calculatedfor the sample.

The term “apparent specific gravity” is the weight per unit volume(grams per cubic centimeter) of the particles, including the internalporosity. The apparent specific gravity for a variety of commerciallyavailable proppants is given in Table I.

TABLE I Commercial Bulk Density Apparent Specific Proppants (g/cc)Gravity (g/cc) 20/40 High Density Ceramic 2.02 3.50 20/40 IntermediateDensity Ceramic 1.88 3.25 20/40 Lightweight Ceramic 1.57 2.71 20/40Resin Coated Sand 1.60 2.61 20/40 Ottawa Sand 1.60 2.62

The term “bulk density” is the weight per unit volume (grams per cubiccentimeter) including the void spaces between the particles in thevolume considered. The bulk density for a variety of commerciallyavailable proppants is given in Table I.

Generally, the production method of this invention involves 7 steps: rawmaterial calcination, blending & grinding, pellet formation, drying &preheating, sintering, cooling and final screening. The following is theprocess description in detail.

Raw Material Calcination

Prior to formation of the pellets, kaolin can be either calcined oruncalcined. Porcelain clay and enthenware clay do not need to becalcinated. If kaolin is calcined, the calcination should be performedat the temperature about 1100° C. The calcination temperature is so highthat it is not just a thermal treatment process in order to bring abouta thermal decomposition, like removing organics, and driving off thewater of hydration and moisture, but also a phase transformation processto undergo to form mullite or crystobalite.

Blending & Grinding

The kaolin shall be blended with uncalcinated and milled porcelain clayand earthenware clay. The alumina content of mixture shall be less than35%. When the mixture having an alumina content less than 35%, it isground into a mean particle size of approximately 3 to 5 microns to forma dry powder prior to formation of the pellets.

Pellet Formation

Various methods of pellet formation can be used, and are well known inthe proppant manufacturing industry, e.g. a method is described in U.S.Pat. No. 4,522,731 using a spray pelletizer or fluidizer. Here, wedescribe a method using mixer as below.

A mixing machine, or mixer, obtained from Yixing Zhengda CeramicEquipment, Inc, can rotate at a speed of from about 10 to 120revolutions per minute (rpm). Then, additional ground and dry claypowder is continually added and mixed, forming spherical pellets untilthe desirable size of green pellets is obtained. Only when the rightamount of water and powder are added under the right rate and righttime, the process can yield smooth, spherical, well-compacted, solidproppant green pellets.

According to this invention, after sintering, the proppant particles aresolid, spherical, and homogeneous in color.

Surprisingly and unexpectedly, it has been found that, to make theproppants of this present invention, it is not necessary to add anybinding materials, or binders. The roundness and sphericity can beachieved at least about 0.8 based on the Krumbein/Sloss Roundness andSphericity Chart.

Drying & Preheating

The spherical pellets are next discharged from the mixer and dried to afree moisture content of less than 3%. The suitable drying temperatureis about 300° C. to about 400° C. and the time is for 30 minutes to 45minutes. Once dried, the desired size spherical pellets are separated byscreening to different sizes. The pellets of undesirable sides will bescreened out and recycled.

Sintering

Soon after the dried green pellets are screened in the desired sizerange, they will be sintered in a rotary kiln or a tunnel kiln. Thesintering time and temperature for rotary or tunnel kiln are critical.There are several variables, including raw material mineralogy, milledparticle size, pellet size and the volume of material in the kiln, todecide the sintering time and temperature.

1) Sintering via Rotary Kiln

Generally, the firing temperature is from about 1150° C. to about 1380°C., preferably between 1200° C. to 1320° C. The firing time is fromabout 75 minutes to about 960 minutes, preferably from about 120 minutesto about 720 minutes.

It has been found, for instance, that the strength of particle withfiring time of 120 minutes is greater than the strength of particle withfiring time of 35 minutes.

Also, applicants found in accordance with this invention that if thefiring temperature is too high, e.g. over 1450° C., the particles willbe poorly shaped and pellets will stick together due to the low alumina(Al₂O₃) content.

2) Sintering via Tunnel Kiln

The tunnel kiln is a car pushing type continuous kiln consists ofpreheating, drying, firing and cooling zones. It is suitable for a largeproduction of ceramic proppants. It is a flow line production method andthe truck of loaded green pellet is loaded every hour or so into one endand removed from the other. A pusher device gradually pushes the newlyintroduced truck and a row of others in front of it—one truck length andthen retracts to permit another load to be charged. The firing zone isat the centre of the tunnel and is maintained at temperature for weeks,months or years, or until maintenance requires a shutdown.

There are several advantages of proppant production in a tunnel kilnthan in a rotary kiln. First of all, the processes of preheating,drying, firing, cooling are happened in a tunnel, a closed loop system.Such tunnel structure absorbs and keeps a lot of heat, and it is easierto set up a temperature and control the firing time. Secondly, since itis a continuous operation, the proppant quality is stable and easy to beimproved. Thirdly, because the pellet is almost in “static” positionduring the heating and firing, the particle is able to keep smooth,spherical, and good shape. Finally, the production is more flexible thanrotary kiln and can be adjust from monthly volume 3,000 metric ton tomonthly volume 6,000 metric ton just according to market demand.

Similar to rotary kiln process, the preheating and drying temperature isabout 300° C. to about 400° C. and the time is about 30 minutes to 45minutes.

After pellet is dried and preheated, it enters into the firing zone thatis at the centre of the tunnel. Generally, the firing temperature isfrom about 1150° C. to about 1380° C., preferably between 1200° C. to1320° C. The firing time is from about 75 minutes to about 960 minutes,preferably from about 120 minutes to about 720 minutes. As a result, theparticle is fully densified and well-shaped.

Also, applicants found that if the firing temperature is too high, e.g.over 1450° C., the particles will be poorly shaped and pellets willstick together due to the low alumina (Al₂O₃) content.

Cooling and Screening

After sintering in rotary kiln or tunnel kiln, the pellets are cooleddown to air temperature about 28° C. and again screened to the desiredfinal sizes, such as 20/40, 40/70 mesh sizes.

After sintering, the pellets are again screened to the desired finalsize. For example, a typical size is 40/70 mesh which contains 90 w/o ofits pellets of between 0.425 μm and 0.212 μm in size.

The proppants of the preferred embodiment of the present invention haveSpecific Gravity (oil per ISO 13503-2) of between 2.10 to 2.55 g/cc,preferably between 2.30 to 2.50 g/cc, and Bulk Density of between 1.30to about 1.50 g/cc, preferably between 1.30 to 1.40 g/cc.

The proppant particles of this invention are homogeneous in color.

The invention is further illustrated by reference to the followingnon-limiting examples wherein all percentages are by weight unlessotherwise specified.

Example 1

A mixture consisting of 52% of porcelain and 26% kaolin were added to22% of earthenware clay powder. Here, the kaolin has been calcined andground. And then mixed with powder of uncalcined porcelain andearthenware clays. All raw materials were minded from Wanyao, Ningde,Fujian province in China.

As an example of this invention, the resulting mixture has an aluminacontent of 23.36%. The detail composition is as follows.

Chemistry Mixture (%) Al₂O₃ 23.36 SiO₂ 70.68 Fe₂O₃ 2.04 K₂O 2.11 TiO₂1.16 MgO 0.30 CaO₂ 0.15 Na₂O 0.18 Others 0.02

The resulting mixture was granulated in a mixer provided by YixinfgZhengda Ceramic Equipment, Inc. The mixer speed was about 25 rpm, andthe total granulation time was about 75 minutes for 40/70 mesh size.

The resulting granulated material was dried in the heater (or dryer) attemperature of 350° C., and then was sintered at a temperature of 1275°C.

The resulting proppant of Example 1 was 40/70 mesh in size, and has beentested by Stim-Lab, a Core Laboratories Company, located in Duncan,Okla. The testing results of Stim-Lab exhibited a bulk density of 1.34g/cm³ and, a specific gravity of 2.42 g/cm³, and a crushing strength of2.7% fines at 7,500 psi, and 6.7% fines at 10,000 psi.

The following is summary of proppant properties in 40/70 mesh sizeevaluated by STIM-Lab, and the test procedure is based on ISO 13503-2:

ITEM TEST RESULT UNIT Sphericity 0.9 n/a Roundness 0.9 n/a AcidSolubility 4.0 % Turbidity 18 FTU Bulk Density 1.34 g/cm³ SpecificGravity 2.42 g/cm³ Crush Test @ 7,500 psi 2.7 % Crush Test @ 10,000 psi6.7 %

Stim-Lab has developed testing techniques for determining theconductivity and permeability of proppant that are widely accepted inthe industry.

After evaluated by STIM-Lab, it has been found that the proppants ofthis present invention have conductivity that is higher than Sands whichhave higher specific gravities. For example, the conductivity of the40/70 proppant of the preferred embodiment of the present invention areat least 560 md-ft, and preferably at least 580 md-ft, after 50 hours at8000 psi and 275° F., with 2% KCI and deoxygenated with nitrogen, asmeasured by STIM-Lab (“STIM-Lab Method”).

The following are procedures for conductivity and liquid permeabilitymeasurements for this invention:

-   -   1. The equipment used for the measurement of conductivity and        liquid permeability included:        -   75 ton Dake Press with air oil intensifier. API SS316 or            Monet K-5000 flow cells with 10 sq in. flow paths.        -   Rosemont (smart family) 40:1 pressure transducers for            measuring pressure drop and reate plumbed with ¼ in. lines            and calibrated with the smart system computer and set at the            0-5 inch of water span range.        -   Two gallon nitrogen driven fluid reservoirs filled with 2%            KCI and deoxygenated with nitrogen.        -   Internal gauges and calipers for measuring widths.        -   IBM PC to process data and calculate conductivity and            permeability.        -   Two—10 sq in. Ohio Sandstones.        -   An API cell was loaded with proppant sample to be tested.            The proppant was leveled with a blade device.    -   2. The proppant sample was placed between the core slabs and was        made a part of a four-cell stack.    -   3. The cells were stacked to within 0.002 in. from top to bottom        and positioned between the platens of the Dake Press. Pressure        was increased to 500 psi and the system was evacuated and        saturated with water at 70-75° F.    -   4. Once saturated, the closure pressure was increased to 1,000        psi, at a rate of 100 psi/min the proppant was allowed to        equilibrate as outlined in the data tables.    -   5. The flow rate, pressure differential, and average width were        measured at each pressure in order to calculate conductivity and        permeability. Five measurements were taken and averaged to        arrive at each conductivity. Flow rate was measured with a        LiquiFlow meter, which was calibrated with a Mettler balance to        0.01 ml/min Darcy's law was used for the calculations to        determine the conductivity and permeability.    -   6. The test temperature was increased to 250° F. and allowed to        equilibrate. The temperature was left at 250° F. for 12 hours        prior to increasing the closure.    -   7. The conductivity and permeability of the proppant were        collected at 1,000 psi at both room temperature and 250° F. as        stated in the data tables.    -   8. The pressure was increased at 100 psi per minute at 1,000 psi        increments and the above measuring technique repeated.    -   9. The conductivity and permeability of the proppant were        continuously monitored at 2,000 psi and 250° F. for 50 hours.    -   10. The conductivity and permeability of the proppant were        continuously monitored at 4,000 psi and 250° F. for 50 hours.    -   11. The conductivity and permeability of the proppant were        continuously monitored at 6,000 psi and 250° F. for 50 hours.    -   12. The conductivity and permeability of the proppant were        continuously monitored at 8,000 psi and 250° F. for 50 hours.

For example, the conductivity of the 40/70 proppant of the preferredembodiment of the present invention are at least 560 md-ft, andpreferably at least 580 md-ft, after 50 hours at 8000 psi and 275° F.,with 2% KCI and deoxygenated with nitrogen, as measured by STIM-Lab(report No. SL 8674).

Hours at Perme- Closure & Closure Temperature Conductivity Width abilityTemperature (psi) (° F.) (md-ft) (in) (Darcy) 50 2000 250 1925 0.253 9150 4000 250 1534 0.248 74 50 6000 250 1085 0.242 54 50 8000 250 5830.233 30 Remarks: 40/70 Ultra-Lightweight Ceramic Proppant. ProppantConcentration 2.0 lb/ft2. Between Ohio Sandstone Core. Evaluated bySTIM-Lab using the STIM-Lab Method.

Example 2

The same raw materials and production method were repeated as describedin Example 1. This time the proppant mesh size was 30/50 which contains90 w/o of its pellets of between 0.600 μm and 0.300 μm in size. Toobtain the green pellets in desirable size, the mixer speed was about 25rpm, and total granulation time was about 90 minutes.

The following is summary of 30/50 proppant properties evaluated bySTIM-Lab, and the test procedure is based on ISO 13503-2:

ITEM TEST RESULT UNIT Sphericity 0.8 n/a Roundness 0.8 n/a AcidSolubility 4.0 % Turbidity 8 FTU Bulk Density 1.31 g/cm³ SpecificGravity 2.41 g/cm³ Crush Test @ 7,500 psi 7.1 %

The same Stim-Lab conductivity & property tests described in Example 1was repeated on the proppant in 30/50 mesh size.

Hours at Perme- Closure & Closure Temperature Conductivity Width abilityTemperature (psi) (° F.) (md-ft) (in) (Darcy) 50 2000 250 3216 0.256 15150 4000 250 2350 0.249 113 50 6000 250 1378 0.239 69 50 8000 250 6570.226 35 Remarks: 30/50 Ultra-Lightweight Ceramic Proppant. ProppantConcentration 2.0 lb/ft2. Between Ohio Sandstone Core. Evaluated bySTIM-Lab using the STIM-Lab Method.

Example 3

A mixture consisting of 65% of porcelain clay and 15% kaolin were addedto 20% of earthenware clay. Here, the kaolin powder was uncalcined. Asan example of this invention, the resulting mixture has an aluminacontent of 19.05%. The detail composition is as follows.

Chemistry Mixture (%) Al₂O₃ 19.05 SiO₂ 71.60 Fe₂O₃ 2.23 K₂O 2.08 TiO₂1.98 MgO 1.93 CaO₂ 0.45 Na₂O 0.23 Others 0.45

The same production method was repeated as described in Example 1. Theresulting proppant of Example 3 was 40/70 mesh in size, and has beenevaluated by the Product Quality Supervision & Inspection Center ofPetroChina. The testing results of PetroChina exhibited a bulk densityof 1.30 g/cm³ and, a specific gravity of 2.40 g/cm³, and a crushingstrength of 4.6% fines at 7,500 psi.

The following is summary of proppant properties tested by PetroChina,and the test procedure is based on ISO 13503-2:

ITEM TEST RESULT UNIT Sphericity 0.88 n/a Roundness 0.88 n/a AcidSolubility 6.0 % Bulk Density 1.30 g/cm³ Specific Gravity 2.40 g/cm³Crush Test @ 7,500 psi 4.6 %

Here is conductivity test evaluated by PetroChina:

Closure Pressure Conductivity (MPa/psi) (md-feet) 10/1450 1072 20/2900938 30/4350 748 40/5800 601 50/7250 367 60/8700 236 Remarks: 40/70Ultra-Lightweight Ceramic Proppant. Proppant Concentration 2.0 lb/ft2.Between Ohio Sandstone Core. Evaluated by PetroChina.

Example 4

The same raw materials and production method was repeated as describedin Example 3. The resulting mixture still has an alumina content of19.05%. This time the proppant mesh size was 20/40 which contains 90 w/oof its pellets of between 0.850 μm and 0.425 μm in size. To obtain thegreen pellets in desirable size, the mixer speed was about 25 rpm, andtotal granulation time was about 105 minutes.

The following is summary of 20/40 proppant property data:

ITEM TEST RESULT UNIT Sphericity 0.9 n/a Roundness 0.9 n/a AcidSolubility 4.2 % Bulk Density 1.30 g/cm³ Specific Gravity 2.40 g/cm³Crush Test @ 5,000 psi 3.8 % Crush Test @ 7,500 psi 9.5 %

Example 5

A mixture consisting of 24% of porcelain clay and 59% flint-clay wereadded to 12% of earthenware clay and 5% of titanium concentrate asadditive. Here, the titanium concentrate can be replaced by zircon sand,aluminum oxide or their combinations.

As an example of this invention, the resulting mixture has an aluminacontent of 34.18%. The detail composition is as follows.

Chemistry Mixture (%) Al₂O₃ 34.18 SiO₂ 57.23 TiO₂ 3.38 Fe₂O₃ 2.08 K₂O1.01 MgO 0.47 CaO₂ 0.40 Na₂O 0.91 Others 0.34

The same production method was repeated to make proppant in 40/70 meshas described in Example 1. The resulting proppant of 40/70 exhibited adensity of 1.40 g/cm³ and, a specific gravity of 2.46 g/cm³, and acrushing strength of 2.36% fines at 7,500 psi, and 5.80% fines at 10,000psi.

The following is summary of 40/70 proppant property data:

ITEM TEST RESULT UNIT Sphericity 0.8 n/a Roundness 0.8 n/a AcidSolubility 5.1 % Turbidity 15 FTU Bulk Density 1.40 g/cm³ SpecificGravity 2.46 g/cm³ Crush Test @ 7,500 psi 2.36 % Crush Test @ 10,000 psi5.8 %

It was observed that after adding the strengthening additives, the 40/70mesh size has less than about 3% crush rate at 7500 psi, and less thanabout 6% crush rate at 10000 psi.

Example Summary

The proppant particles of example 1 to 5 are homogeneous in color. Theproperty data of test results are as follows:

Exam- Exam- Exam- Exam- Exam- ITEM ple 1 ple 2 ple 3 ple 4 ple 5Proppant API Size 40/70 30/50 40/70 20/40 40/70 Alumina Content (%)23.36 23.36 19.05 19.05 34.18 Silicon Dioxide Content 70.68 70.68 71.6071.60 57.13 (%) Sphericity 0.9 0.8 0.88 0.9 0.8 Roundness 0.9 0.8 0.880.9 0.8 Acid Solubility (%) 4.0 4.0 6.0 4.2 5.1 Bulk Density (g/cm3)1.34 1.31 1.30 1.30 1.40 Specific Gravity (g/cm3) 2.42 2.41 2.40 2.402.46 Crush Test @ 5,000 psi — — — 3.8 — (%) Crush Test @ 7,500 psi 2.77.1 4.6 9.5 2.36 (%) Crush Test @ 10,000 psi 6.7 — — — 5.8 (%)

The proppant disclosed in this invention in 40/70 mesh size has lessthan about 5% crush rate at 7500 psi, and less than about 10% crush rateat 10000 psi.; in 30/50 mesh size less than about 10% crush rate at 7500psi, and less than about 15% at 10000 psi; and in 20/40 mesh size lessthan about 15% crush rate at 7500 psi, and less than about 20% crushrate at 10000 psi.

It will be obvious to those skilled in the art that the inventiondescribed here can be essentially duplicated by making minor changes inthe material content or the method of manufacture. To the extent thatsuch material or methods are substantially equivalent, it is intendedthat they be encompassed by the following claims.

What we claim is:
 1. A pellet made from a mixture of natural occurringclays comprising porcelain clay, earthenware clay, and kaolin clay;wherein each of said clay having an alumina content and a silicondioxide (SiO₂) content.
 2. The pellet of claim 1, wherein said porcelainclay possesses a weight percentage of at least 5% in said mixture. 3.The pellet of claim 1, wherein the alumina content of said pellet isless than 35%.
 4. The pellet of claim 3, wherein the alumina content ofsaid pellet is more than 5.5%.
 5. The pellet of claim 4, wherein saidpellet has a silicon dioxide (SiO₂) content between about 89.5% and69.5%.
 6. The pellet of claim 1, wherein said pellet has an apparentspecific gravity from about 2.10 g/cc to about 2.55 g/cc, and a bulkdensity from about 1.30 g/cc to about 1.50 g/cc.
 7. The pellet of claim1, wherein said pellet exhibits greater conductivity than sand atpressures up to 8,000 psi after 50 hours and 275° F. on Ohio Sandstone,in the presence of deoxygenated aqueous 2% solution of KCI as measuredby the Stim-Lab Method.
 8. The pellet of claim 1, wherein the pellet in40/70 mesh size has less than about 5% crush rate at 7500 psi, and lessthan about 10% crush rate at 10000 psi.
 9. The pellet of claim 1,wherein the pellet in 30/50 mesh size has less than about 10% crush rateat 7500 psi, and less than about 15% crush rate at 10000 psi.
 10. Thepellet of claim 1, wherein the pellet in 20/40 mesh size has less thanabout 15% crush rate at 7500 psi, and less than about 20% crush rate at10000 psi.
 11. The pellet of claim 1, wherein said pellet consists ofsolid spherical particles that are homogeneous in color.
 12. The pelletof claim 1, wherein said natural occurring clay mixture furthercomprises a strengthening additive selected from the group consisting oftitanium concentrate, zircon sand, aluminum oxide or combinationsthereof.
 13. A method of fracturing a subterranean formation whichcomprises injecting a hydraulic fluid into the formation at a rate andpressure sufficient to open a fracture therein, and injecting into thefracture a fluid containing the pellet of claim 1.